I will be creating a tutorial especially for Twilight, but for now there is one old tutorial I made that relate to this topic perfectly.
a PDF version step-by-step what to do with the alpha mask once you have itpg8 of the Twilight User Manual 'Advanced Settings' section wrote:
- Select an object.
- Choose “Alpha Mask” as your render method. (Camera Tab: Be sure exposure and gamma = 1.0)
- Start the rendering.
- When asked to render only selected objects choose “yes”.
- A black and white image will be created of the object.
- Save the rendered mask image.